1. Field of the Invention
The invention relates to an Oldham coupling to connect two parallel structural parts, for example shafts, wherein the intermediate ring freely moving between the hubs of the two structural parts to be connected carries on its flat sides two lands located at 90.degree. relative to each other, said lands engaging corresponding grooves in the hubs. Oldham couplings are especially suitable as the coupling element for a device revolving on a path, in which two structural parts must be maintained in a predetermined angular relationships relative to each other. This is the case for example in apparatuses, such as rotating piston positive displacement machines.
2. Discussion of Related Art
Oldham couplings of the above-mentioned type are disclosed in EP 10930 Bl, U.S. Pat. No. 4,437,820 and DE 27 35 664. All of those disclosures relate to displacement machines for compressible media. They each comprise a working chamber defined by helical circumferential walls extending vertically from a side wall and leading from an inlet located outside the helices to an outlet inside the helices. They further contain a helical displacement body extending into the working chamber. The latter is supported rotatingly without rotation relative to the working chamber. Its center is eccentrically offset relative to the center of the circumferential walls, so that the displacement body is always in contact with both the outer and the inner circumferential wall of the working chamber along at least one advancing line. During the operation of the machine therefore a plurality of sickle shaped working spaces are enclosed. The working spaces move from the inlet to the outlet through the working chamber. Depending on the angle of contact of the helix, the volume of the working medium conveyed may be gradually reduced with a corresponding increase of the pressure of said medium.
In the known machines, the relative rotating motion is always transmitted by a highly stressed and thus expensive ball bearing. Furthermore, no measure is provided to insure the operation without clearance of the machine in case of the wear of the material of the wobble rod or rods. In all of those known machines, Oldham (cross-keyed) couplings are the rotation inhibiting means for the displacer. Radial displacement is limited by the contact of the helical ribs with the walls of the working chambers. This limitation theoretically corresponds to a circle, in this case a translational circle. The displacer, which does not rotate relative to the working chamber, must be guided by means of the Oldham coupling in a manner such that the parallel guidance permits a larger diameter than that corresponding to the diameter of the translation circle. The reason for this is the fact that the radial displacement of the displacer is to be limited by the rib/chamber wall combination and not by the guiding Oldham coupling. Using this rule, the dimensions of the Oldham coupling are readily determined.
It is generally believed that such Oldham couplings are not suitable for the transmission of large torques and high rpm in view of the bending fatigue exposure and frictional losses. As in operation the lands are constantly sliding back and forth in the grooves, the Oldham coupling should be running in an oil bath in order to reduce friction losses and wear. Thus for example, the lands of the Oldham coupling according to DE 27 35 664 include, although they consist of a self-lubricating material such as polyimide, regular oil grooves in their lateral sliding surfaces, intended to promote the development of an effective and efficient oil film around the lands.
In all of the known Oldham couplings the lands consist of rectangular blocks engaging correspondingly shaped grooves. The objections to the use of Oldham couplings is understandable to the extent that the lateral clearance in the grooves must be minimal for uniform guidance. However, this necessarily leads to frictional surfaces which tend to wear. In addition, dirt may penetrate the coupling and jam the parts, which interferes with the operation of the coupling.